[XFS] replace dquot flush semaphore with a completion

Use the new completion flush code to implement the dquot flush lock.
Removes one of the final users of semaphores in the XFS code base.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31822a

Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>